Printing an image on a substrate

ABSTRACT

Printing a screened image includes a computer for converting a contone image to a digital image. A digital front end provides a screening tile, a dither magnitude curve, a spot function for each of the cells in the screening tile and a displacement vector for each of the cells. The digital front end displaces the center of each cell spot function and constructs a halftone image from the contone image and the screening tile by comparing value of each pixel from the contone image to a corresponding threshold value from the screening tile. If the pixel value exceeds the threshold value set a corresponding pixel in the halftone image to one otherwise set the corresponding pixel in the halftone image to zero. An imaging system prints the halftone image on a substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned copending U.S. patent applicationSer. No. 14/253,905, filed Apr. 16, 2014, entitled FORMING AN IMAGE ON AMEDIA, by Bielak; the disclosure of which is incorporated herein.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for imagereproduction systems characterized by three-dimensional features imagedon a flexographic plate.

BACKGROUND OF THE INVENTION

Halftone screening is a printing technique that reproduces continuoustone images by using dots that vary in size or spacing. Halftonescreening is used by many different printing technologies. The inventiondisclosed here is particularly valuable for screening applied toflexographic printing.

There are two common halftone screening methods used in the industry:Amplitude Modulation (AM) screening and Frequency Modulation (FM)screening. The most common screening method is AM, which positions dotson a fixed grid and varies the dot size according to the tone of theimage. Various dot shapes can be used but round, elliptical, andEuclidian are the most common. FIG. 1A shows a typical AM screen withdots 104 positioned on the grid.

The principal disadvantage of AM screening for flexographic printing isthe size of the smallest printing dot. The smallest relief dot that canbe reliably created on a plate is about 20 to 30 microns in diameter.Any smaller and the dot may not form properly, leading to distortion andscum dot formation. In addition, the ink transferred from the plate tothe substrate spreads out past the edge of the dot, increasing the dotdiameter by another 20 to 30 microns. This results in a high minimumtonal value. For example, a 150 dpi screen will have minimum tonal valueof about 10%.

A second method, often called FM screening, avoids a regular grid.Instead a stochastic process is used to place the dots. Initially thedots are of fixed size and the tone is increased by adding dots to theimage. After a certain density is reached, the algorithm switches fromdot addition to dot growth. The placement algorithm attempts topreferentially reduce the spatial frequency components in the screen towhich the eye is most sensitive. FIG. 1B shows a typical FM screen withdot 108 positioned on the plate.

The main disadvantage of this method is the residual noise that isapparent in the mid-tones of the image. Mid-tones can be roughly definedas the tonal range from about 25% to 75% tone. This noise is often madeworse when different color image separations are superimposed during theprinting process. In this mid-tone range, AM screening usuallyout-performs FM screens.

The principal advantage of FM screens is their ability to achieve lowertonal values, when compared to AM screens, by continually removing dotsfrom the screen. These lower tonal values in the range of zero to a fewpercent are referred to as the highlights. Similarly, the last fewpercent of tonal values up to 100% tone are referred to as the shadows.An ideal halftone screen would have the low noise characteristic of AMscreens in the mid-tones combined with the extended tonal range of FMscreens in the highlights. Several methods described in the prior artattempt to achieve this result, but with mixed results. There exists aneed to create a hybrid screen without introducing any unwanted sideeffects.

In the prior art, the most straight forward method of extending thetonal range of an AM screen is to start removing dots after the minimumdot size has been reached. A stochastic algorithm is used to choose theorder in which the dots are removed. The goal of the algorithm is tominimize noise in the low spatial frequencies of the screen. Someclustering of the remaining dots may be required to prevent scum dotsfrom forming on the plate. FIG. 1C shows how this method is used in thehighlight area of a vignette. Area 112 shows dots are removed from theAM grid. What is apparent from the figure is that the dot dropout causesvoids 112 in the halftone screen that are easily visible. Many customersfind such voids objectionable.

Better results can be achieved when a hybrid of AM and FM screening isused. A stochastic screen is used for the highlights areas while, in themid-tones, an AM screen is deployed for noise reduction. There is anabrupt transition from FM to AM screen at the tone when all the AM cellsare filled with a halftone dot of the minimum size. The algorithmensures that a separation is maintained between AM and FM dots. Thisalgorithm is an improvement over the simpler on-grid algorithm, however,it does sometimes suffer from a visible line appearing between the AMand FM regions. This can manifest as either a white line or a dark linebetween the regions. FIG. 1D shows an example of this type of screen.The voids 116 are not as apparent in the highlights

A further enhancement is to add a transition region between the FMscreening in the highlights and the AM screening over the remainder ofthe tonal range. In this transition region, the screening method slowlychanges from FM to AM and the average position of the halftone dotsmigrate toward the center of the halftone cell. FIG. 1E illustrates anexample of such an implementation.

SUMMARY OF THE INVENTION

Briefly, according to one aspect of the present invention a system ofprinting a screened image includes providing a contone image, ascreening tile structure of M by N cells, a dither magnitude curve, aspot function for each of the cells in the screening tile, and adisplacement vector for each of the cells in the screening tile.Threshold values for each of the cells are constructed by displacing thecenter of each cell spot function according to the displacement vectorsand according to the dither curve. A halftone image is constructed fromthe contone image and the screening tile by comparing value of eachpixel from the contone image to a corresponding threshold value from thescreening tile. If the pixel value exceeds the threshold value set acorresponding pixel in the halftone image to one otherwise set thecorresponding pixel in the halftone image to zero. An imaging systemprints the halftone image on a substrate.

These and other objects, features, and advantages of the presentinvention will become apparent to those skilled in the art upon areading of the following detailed description when taken in conjunctionwith the drawings wherein there is shown and described an illustrativeembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A represents, in diagrammatic form, a prior art AM screen sample;

FIG. 1B represents, in diagrammatic form, a prior art FM screen sample;

FIG. 1C represents, in diagrammatic form, a prior art dot removal froman AM screen;

FIG. 1D represents, in diagrammatic form, a prior art abrupt hybridscreen;

FIG. 1E represents, in diagrammatic form, a prior art transition from FMto AM screen;

FIG. 2 represents, in diagrammatic form, a prior art digital front enddriving an imaging device;

FIG. 3 represents, in diagrammatic form, a prior art laser imaging headsituated on the imaging carriage imaging on a plate mounted on animaging cylinder;

FIG. 4 shows a prior art halftone rendered image;

FIG. 5 shows a prior art rendered image on flexographic plate;

FIG. 6 shows the magnitude of the dither as a function of tonal value ina typical application;

FIG. 7 shows a sample of a screened vignette with the method describedin the invention;

FIG. 8 shows a block diagram of the halftone screening process accordingto the current invention;

FIG. 9 shows a halftone screened tile;

FIG. 10 shows constructed threshold values in a cell of a screeningtile; and

FIG. 11 is a flow chart showing an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the disclosure.However, it will be understood by those skilled in the art that theteachings of the present disclosure may be practiced without thesespecific details. In other instances, well-known methods, procedures,components and circuits have not been described in detail so as not toobscure the teachings of the present disclosure.

While the present invention is described in connection with one of theembodiments, it will be understood that it is not intended to limit theinvention to this embodiment. On the contrary, it is intended to coveralternatives, modifications, and equivalents as covered by the appendedclaims.

FIG. 2 shows an imaging device 208. The imaging device is driven by adigital front end (DFE) 204. The DFE receives printing jobs in a digitalform from desktop publishing (DTP) systems usually comprising a computer40, and renders the digital information for imaging. The renderedinformation and imaging device control data are communicated between DFE204 and imaging device 208 over interface line 212.

FIG. 3 shows an imaging system 300. The imaging system 300 includes animaging carriage 332 an imaging head 320 is mounted, imaging head 320are controlled by controller 328. The imaging head 320 is configured toimage on a substrate 308 (such as film or plate) mounted on a rotatingcylinder 304. The carriage 332 is adapted to move substantially inparallel to cylinder 304 guided by an advancement screw 316. Thesubstrate 308 is imaged by imaging head 320 to form an imaged data 312on substrate 308.

FIG. 4 shows a halftone rendered image 400. The rendered image 400 wasprepared by DFE 204, to be further imaged on the substrate 308. FIG. 5shows rendered image 400 imaged by imaging head 420 on substrate 308forming an imaged substrate 500.

For halftone screens in flexographic printing, the best results areachieved if the highlights areas 604 are predominantly screened usingstochastic based methods and the dots in the mid-tones 612 area arepredominantly on a regular grid. This is the objective of the currentinvention.

The present invention comprises an AM screen wherein the center of thehalftone dot is dithered from its central location. The magnitude of thedither is a function of the tonal value. FIG. 6 shows the magnitude ofthe dither as a function of tonal value in a typical application. Thecurve can take many shapes but is generally high in the highlight areas604 and low in the mid-tone areas 612. A dashed line 616 is shownextending the low value into the shadows 608. Alternatively, the shadowareas 608 can also be highly dithered since they suffer from many of thesame problems as the highlights 604.

The magnitude of the dither in the highlight and shadows approaches100%. This implies that the center of the dot can be located almostanywhere in the halftone cell. As such, the spatial frequency responseof the screen is very similar to that of an FM screen. The dotdisplacement vectors are chosen to minimize low frequency spatialcomponents in the screen.

The mid-tones contain a small amount of dither. A slight dither to theposition of the dot center in the halftone spot function can preventdots from touching at the same time in a tonal vignette. When dots touchthere is often a step change in the amount of ink laid down. This cancause a visible artifact in the image. Smearing the tone at whichcontact occurs can mitigate this problem. The dither can also break upany periodic patterns that can occur in the shape or size of thehalftone dot during the screening process. Such repeat patterns maycause moiré artifacts to appear in an image.

The prior art uses one type of screening method in the highlights (andshadows) and another screening method in the mid-tones. By contrast, thepresent invention uses the same screening method throughout the entiretonal range. Only the magnitude of the applied dither changes as afunction of tonal value. FIG. 7 shows a sample of a screened vignettecreated with the method described in the invention.

In the preferred embodiment, the FM screen is constructed as a screeningtile 900 with a width and length that is an integral number of halftonecells 912 as shown in FIG. 9. The tile is also composed of an integralnumber of pixels 904. These pixels are the smallest addressable pictureelements of the imaging device. The halftone dots 908 within thehalftone cells are formed by setting adjacent groups of pixels to theone state from the zero state. (On the printing plate, set pixelstransfer ink to the substrate, whereas, clear pixels do not). Typically,the tile is not large enough to cover the entire area of the halftoneimage and the tile must be repeated, in both dimensions, to achieve thedesired coverage.

The FM screen is constructed using the following procedure. One minimumsize halftone dot is randomly placed in each halftone cell of the tile.This initial placement creates a tile with a white noise characteristicthat contains visibly objectionable artifacts. To improve the screen,the position of the dots are adjusted in an iterative manner to reducethe low frequency components while suppressing the tendency of the dotsto migrate toward the center of the halftone cell. The optimizationalgorithm takes into account the repeating nature of the tile to ensurethat no artifacts are created at tile boundaries.

Because the dots are restricted to staying within the boundaries oftheir halftone cell in the present invention, the resulting screen isnot a pure FM screen. FM screens, on average, will have one dot per cellbut an individual cell may contain 0, 1, 2, or more dots. For thisreason, the frequency response of screen tile will always retain a smallAM component. From the resulting FM tile, a set of displacement vectorsare extracted. These vectors are the distance and direction that eachhalftone dot is displaced from the center of its corresponding halftonecell.

Threshold values can now be constructed for each cell in the screeningtile from the displacement vectors, the dither magnitude curve (FIG. 6),and a desired spot function. The screening tile containing thresholdvalues is a common method of implementing a halftone screen. Each pixelin the continuous tone image is compared with a corresponding thresholdvalue in the tile. If the image value exceeds the threshold value thenthe corresponding pixel in the halftone image is set. FIG. 10 shows howthreshold values in a cell of a screening tile 1000 might be configuredfor a single halftone cell 912 in the screening tile 900. The othercells in the tile would be configured similarly. These threshold valuesare compared against the tonal values of the corresponding pixels in thecontone image and if the threshold value is less than or equal to theimage value then the corresponding pixel in the halftone image is set.FIG. 9 shows the result of the thresholding operation for the case wherethe contone image area that the tile covers is a 19% tone as is shown bynumeral 1004 (12 of the 64 pixels in each halftone cell are set)

A conventional AM screen threshold values for a cell in a screening tileis created by first selecting a spot function. A spot function definesthe order that pixels are turned on within a halftone cell controllingthe dot shape. One common spot function produces a circular dot over theentire tonal range, another function transitions from a circular shapeto a checkerboard at the midtone and back to circular in the shadows.The dots start growing from the center of the halftone cell and remaincentered on the middle of the cell. The threshold values for a singlecell are created by selecting for each successive threshold value thenext pixels as defined by the spot functions and inserting the thresholdvalue into those locations.

The current invention departs from the conventional AM arrayconstruction method by the use of the direction vectors and the dithermagnitude curve (FIG. 6) to modify the spot function. The thresholdvalues for a cell are created by selecting for each successive thresholdvalue the next pixels in the contone image as defined by the modifiedspot functions and inserting the threshold value into those locations.The modified spot functions are derived from the original spot functionsby displacing the center of the original spot function from the centerof the halftone cell using values calculated from the product of thedisplacement vectors and the dither magnitude curve.

In a conventional AM screen, as the tone increases, pixels are set andstay set. This requires only one threshold value per location in thetile. Because the magnitude of the displacement of the halftone dotschanges as a function of tonal value in the current invention, pixelsmay toggle from zero to one and back to zero again. Therefore,multilevel threshold values are implemented for this invention.

FIG. 8 depicts a flow diagram to generate a halftone image according tothe current invention. Cell threshold values 804 are constructedaccording to the extracted displacement vectors 812 from the FM tile andthe dither magnitude curve 808. A halftone image 824 will be generatedfrom a supplied contone image 820 by comparing the constructed thresholdvalues 816 elements to the contone image pixels.

FIG. 11 is a flow chart which shows a system of printing a screenedimage includes providing a contone image 1100, a screening tilestructure of M by N cells, a dither magnitude curve, a spot function foreach of the cells in the screening tile, and a displacement vector foreach of the cells in the screening tile 1102. Threshold values for eachof the cells are constructed by displacing the center of each cell spotfunction according to the displacement vectors and according to thedither curve 1104. A halftone image is constructed from the contoneimage and the screening tile by comparing value of each pixel from thecontone image to a corresponding threshold value from the screening tile1106. If the pixel value exceeds the threshold value set a correspondingpixel in the halftone image to one otherwise set the corresponding pixelin the halftone image to zero 1108. An imaging system prints thehalftone image on a substrate 1110.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

-   40 computer-   104 AM dots placement-   108 FM dots placement-   112 void dots-   116 void dots-   204 digital front end (DFE)-   208 imaging device-   212 interface line-   300 imaging system-   304 rotating cylinder-   308 substrate-   312 imaged data on substrate-   316 screw-   320 imaging head-   328 controller-   332 carriage-   400 rendered halftone image to be imaged on substrate-   500 rendered image imaged on substrate-   604 highlights-   608 shadows-   612 mid-tones-   616 low values extending into shadows-   804 a cell threshold values construction-   808 dither magnitude curve-   812 displacement vectors-   816 screening component according to threshold values-   820 contone image-   824 halftone image-   900 screening tile-   904 pixel-   908 halftone dot-   912 halftone cell-   1000 a cell threshold values-   1004 pixels under 19% tone will be set-   1100 computer converts contone image to digital image-   1102 screening tile structure of M by N cells-   1104 constructing threshold values for each cell-   1106 constructing halftone image from contone image and screening    tile-   1108 if pixel value exceeds threshold value set corresponding pixel    in halftone image-   1110 imaging system prints halftone image on substrate

The invention claimed is:
 1. A system for printing a screened imagecomprising: a computer for converting a contone image to a digitalimage; a digital front end for providing a screening tile structure of Mby N cells, a dither magnitude curve, a spot function for each of thecells in the screening tile, and a displacement vector for each of thecells in the screening tile structure of M by N cells; wherein thedigital front end constructs threshold values for each of the cells bydisplacing the center of each cell spot function according to thedisplacement vectors and according to the dither magnitude curve;wherein the digital front end constructs a halftone image from thecontone image and the screening tile structure of M by N cells bycomparing value of each pixel from the contone image to a correspondingthreshold value from the screening tile structure of M by N cells; ifthe pixel value exceeds the threshold value set a corresponding pixel inthe halftone image to one otherwise set the corresponding pixel in thehalftone image to zero; and an imaging system for printing the halftoneimage on a substrate.
 2. The system of claim 1 wherein each halftone dotmoves toward the center of the each cell as each halftone dot grows. 3.The system of claim 1 wherein each halftone dot remains entirely withinthe cell boundaries as it grows.
 4. The system of claim 1 wherein amagnitude of dithering is greater in highlight areas than in mid-toneregions.
 5. The system of claim 1 wherein the tile is smaller than thehalftone image.
 6. The system of claim 5 wherein the tile is replicatedthroughout the digital image to form a screened image.
 7. The system ofclaim 1 wherein dithering is adjusted at tile boundaries to reduceartifacts when a tile is replicated.
 8. The system of claim 1 whereinthe stochastic method is optimized to produce displacement vectors thatminimize low frequency noise.
 9. The system of claim 1 wherein M is aninteger.
 10. The system of claim 1 wherein N is an integer.
 11. Thesystem of claim 1 wherein N equals to M.
 12. The system of claim 1wherein N is not equal to M.